Fix knot class loading allowlist not working

[LostLuma]

Resolves #251.

The method of looking up which prefixes were (dis)allowed was flawed as it always looked up using the entire path to a class file (including the part in the jar), whereas the allowlist only maps from the path to the jar to allowed prefixes in the jar.

I've also changed UrlUtil.asUrl to normalize the path as otherwise it produces URLs with a full stop at the current working directory, which caused issues with looking up the prefixes as well. See an equivalent example:

- Path.of("./src/hello.txt").toUri().toURL();
- $1 ==> file:/C:/Users/lilly/projects/quilt-loader/./src/hello.txt
+ Path.of("./src/hello.txt").toAbsolutePath().normalize().toUri().toURL();
+ $2 ==> file:/C:/Users/lilly/projects/quilt-loader/src/hello.txt